Method for the development of hard coat seal surfaces

ABSTRACT

A method for imparting wear and corrosion resistance to valve closure elements which includes depositing a hard facing alloy coating on the metal substrate surface of the valve element followed by a series of heating steps to fuse the alloy coating and age harden the metal substrate.

United States Patent Tiner et al. Feb. 15, 1972 [54! METHOD FOR THE DEVELOPMENT OF [56] References Cited T HARD COA SEAL SURFACES UNITED STATES PATENTS 72l tzNlh A.T' L B h; d l 1 T Z' g sj gz fi a 2,864,696 12/1958 Foreman ..111/105 x 3,322,580 5/1967 Haynes ...14s/12.3 x [73] Assignee: The Uni stats f Amelia! 85 3,378,392 4/1968 Longo ..ll7/93.1

represented by the Secretary of the Air Force Primary Examiner-Ralph S. Kendall [22] Filed: Mar. 21, 1969 Att0rneyHarry A. Herbert, Jr. and William J. OBrien [21] Appl. No.: 809,370 [57] ABSTRACT A method for imparting wear and corrosion resistance to valve [52] US. Cl ..ll7/93.l, 1 17/ 105. l 1 17/1 19, lo re el ments which includes depositing a hard facing alloy 143/ 142 coating on the metal substrate surface of the valve element [51] lllt. Cl ..C23C 7/00 f fl d b a rie of heating steps to fuse the alloy coating [58] Field of Search ..117/93.l, 105, 105.1, 105.2, d age harden h metal b t t I 1 Claim, N0 Drawings BACKGROUND OF THE INVENTION materials presently used in fabricating valve elements do not possess the necessary erosion, wear and corrosion resistance needed to overcome the corrosive impact of presently known propellant fuels, especially those with a fluorinebase. Previous attempts to improve their performance in valve seal applications involving corrosive fluids have not proved successful since prior protective coatings did not possess the desired combination of erosion, wear and corrosion resistance. Oftentimes, the coatings were too thick and interfered with the sealing functions of the closure element.

DESCRIPTION OF THE PREFERRED EMBODIMENT Pursuant to the above-defined objects of this invention, the present method involves a process of depositing a nickel base or cobalt base hard facing alloy on the metal substrate surface of valve closure elements followed by fusion heating to produce a dense, impervious coating resistant to corrosion and erosion. The hard facing alloy compositions are used in powdered form with a mesh size of about 200 to 320 mesh. The alloy is deposited on the surface ofthe sealing element by plasma spraying using a Giannini Model 50-! Plasmaton with a rated power of KW. and Argon cooling gas. The substrate to be coated is first cleaned by vapor degreasing and grit blasting. The alloy is then deposited into the surface of the sealing element by plasma spraying. The sprayed coating is next fused by annealing in vacuum for different temperatures and'various periods of time to control porosity. After fusion, the coated element is subjected to a heating cycle of l.750 F. for l hour. air cooled, aged at l,350 F. for 8 hours, and furnace cooled to develop optimum mechanical properties in the substrate metal. The detailed compositions of a number of specific hard facing alloys which have been found to be most effective for valve seal applications in accordance with this invention are In attempting to provide valve closure elements ol'sufficient corrosionresistance to be useful in controlling the flow of corrosive propellant fuels, it has been found that a protective coating of a hard facing alloy deposited on the metal substrate surface of a valve closure element effectively overcomes the problems encountered heretofore. The method of the invention involves the deposition of a hard facing alloy by plasma spraying followed by fusion heating to obtain dense and impervious coatings.

SUMMARY OF THE INVENTION In accordance with this invention, valve seal elements are provided with a wearand corrosion-resistant coating by a method which includes plasma spraying a powdered hard facing alloy onto the metal substrate surface of a valve closure element followed by fusion heating and age hardening. The resultant coating produces a hard, dense, and impervious surface which is highly resistant to the corrosive effects of propellant fuels.

The hard facing alloys used in this invention are well known in the art and form no part of this invention. These materials are available in powder form from commercial sources and, in general, comprise more than 150 different compositions. The alloys which have been found to be most effective for the purpose of this invention are the nickel and cobalt base alloys having a high-chromium content.

Accordingly. the primary object of this invention is to provide for the improved performance of valve seal elements in applications involving the flow control ofcorrosive fluids.

Another object of this invention is to provide a method for imparting wear and corrosion resistance to the surface of valve closure elements.

Still another object of this invention is to provide a method for depositing a corrosion-resistant protective coating onto the surfaces of valve seal materials.

The above and still other objects and advantages of the present invention will be readily apparent upon consideration of the following detailed description thereof.

presented in Table I together with their deposit hardnesses as measured by their Rockwell C hardness numbers. I

To demonstrate the details of this invention with greater particularity, flat poppet and seat conflguration-type valve element were selected for coating. The elements were machined to tolerances from an lnconel X-750 metal substrate. The parts had 0.050 inch deep and 0.200 inch wide machined grooves. They were vapor degreased by washing in methylethylketone, grit blasted with 200-mesh silicone carbide and then vapor degreased again.

An 0.070 inch thick overlay of a cobalt base hard facing alloy having the composition of example No. I from Table I was deposited over the grooves by a plasma spray technique using argon for arc, feed and cover gas while the part was rotated. A brazing stop-off was used to prevent the alloy from running into the undesirable areas. The plasma spraydeposited coating was then fused by vacuum annealing. The heating cycle selected for this purpose was (I) heat to 1,200 F. and to 1,600 F., held for l0 minutes to stabilize the temperature, (2) heat to 2,140 F. in incremental steps held for 10 minutes, (3) cool to l,900 F. rapidly, then to l,750 F. in approximately l0 minutes and held for 60 minutes, and (4) break the vacuum and rapidly cool in argon to room temperature.

The coated element, after the fusion cycle, was then subjected to an age hardening treatment to harden the lnconel X-750 metal substrate by holding 8 hours at 1,350" F. furnace cooled to l,l50 F., then air cooled to room temperature. After complete heat treatment, the parts were machined to obtain hard face alloy seal surfaces of about 0.020 inch landing area height, and 0024-0027 inch landing width for poppet, and 0.042-0052 inch for seat.

These poppets and seats were tested by assembling them in a fluorine main shutoff valve of conventional design.

A helium leak test measurement showed l0 cc./hr. leak rates before fluorine exposure, and I00 cc./hr. leak rates after 525 cycles service at -300 F. with p.s.i. upstream pressures. Examination of seal surfaces after fluorine service indicated only slight general etching of the alloy microstructure.

However, the general surface appearance of the poppet and its porosity were found to be good.

The method of this invention has been found to be most effective in improving the performance of valve closure elements likely to encounter the severe corrosive effects of liquid propellant fuels. The dense and impervious coating formed on the metal surfaces of the valve elements imparts a high degree of erosion, wear and corrosion resistance to the sealing elements thereby permitting their use in aerospace applications involving the flow control of highly corrosive liquids.

While the invention has been described with particularity in reference to a specific embodiment thereof, it is to be clearly understood that the disclosure of the present invention is for the purpose ofillustration only and is not intended to limit the invention in any way.

What is claimed is:

l. A method for treating a metallic element to render the surface thereof resistant to corrosion comprising the steps of:

a. cleaning the metal surface ofsaid element;

applying a plasma-sprayed layer ofa powdered, hard-facing cobalt-base alloy of high-chromium content onto said cleaned surface;

. heating said element within a vacuum to 1,200 F. and

l,600 F. and holding for 10 minutes, further heating said element to 2,l40 F. in incremental steps and holding for 10 minutes, cooling said element to l,900 F. rapidly, further cooling said element to l,750 F. over a period of approximately 10 minutes and holding at l,750 F. for 60 minutes to cause fusion of the said alloy powders thereto to form a dense. impervious coating over the surface of said element;

. cooling said heated element to room temperature;

heating said cooled element to a temperature of 1.350" F. for a period of about 8 hours to age-harden the coated element and;

. cooling said element thus treated to provide a metal element having enhanced resistance to surface corrosion. 

